Light emitting diodes are widely used in consumer and commercial applications. As is well known to those having skill in the art, a light emitting diode generally includes a diode region on a microelectronic substrate. The microelectronic substrate may comprise, for example, silicon, gallium arsenide, gallium phosphide, alloys thereof, silicon carbide and/or sapphire. Continued developments in LEDs have resulted in highly efficient and mechanically robust light sources that can cover the visible spectrum and beyond. These attributes, coupled with the potentially long service life of solid state devices, low cost of operation, low heat generation, increased efficiency and other benefits may enable a variety of new display applications, and may place LEDs in a position to compete with the well entrenched incandescent and fluorescent lamps.
In certain applications, solid state lighting has already begun to displace traditional incandescent lighting. Most notably, many municipalities in the U.S. and abroad have started replacing traditional incandescent traffic signal bulbs with solid-state light engines. Although the initial cost of installation is relatively high, LED-based traffic signals have, generally, a substantially longer operational life and a far lower cost per lumen than traditional incandescent bulbs.
The move to LED-based traffic signals (particularly the red lights) was a natural beginning for the penetration of traditional incandescent and fluorescent markets by solid state light sources. High brightness red, yellow and, most recently, green LEDs have become available in the marketplace at a reasonable cost within the last decade. Recently, solid state white light engines have been finding applications in the automotive and cellular telephone industries as backlights for instrument panels, switches and LCD displays. Although the technology is still in its infancy, solid state white LEDs are now commercially available. Solid state white LEDs may be fabricated a number of ways. Current technology for providing solid state white lighting generally falls into one of three categories: color mixing, wavelength conversion and hybrid methods that combine aspects of color mixing and wavelength conversion.
Color mixing involves the synthesis of white light from a combination of light sources emitting complementary colors that combine to produce white light (for example, red, green and blue LEDs, or blue and yellow LEDs). Examples of color mixing are found in, for example, U.S. Pat. No. 6,132,072 and references cited therein. Wavelength conversion refers to the use of light of a first wavelength as the excitation signal to cause emission of light of a second wavelength (usually by a phosphorescent or fluorescent material). For example, a UV light source may be used to excite a phosphor capable of emitting red, green and blue light. The resulting light output of the phosphor is a combination of the three colors which, if properly balanced, appears white. See, e.g. U.S. Pat. No. 6,084,250.
White light may also be produced by methods that may be viewed as hybrids of color mixing and wavelength conversion. For example, a white emitter may be fabricated by coating a blue LED with a phosphor that emits yellow light upon excitation with blue light. The combination of blue light from the LED and excited yellow emission from the phosphor produces white light. Examples of phosphors for white light conversion may be found in U.S. Pat. Nos. 5,998,925, 6,066,681 and 6,013,199, which are hereby incorporated herein by reference. Other methods of producing solid state white light are possible.
Despite the availability of solid state white light sources, the vast majority of the market for white lighting applications (namely, home and office lighting) remains relatively untapped. Part of the reason for this is that LEDs are typically not directly compatible with existing power distribution networks.
Existing power distribution networks provide high-voltage (110V or 220V) low current power to homes and businesses in the form of alternating current (AC). “AC” means that the polarity (i.e. direction) of the supplied current alternates with each cycle. For standard 60 Hz power supplies, this means that the polarity of the current changes 120 times per second.
In contrast, LEDs are low-voltage, high-current devices that by their nature permit current flow in only one direction, and hence, are considered direct current (DC) devices. Thus, efficient power distribution or transformation systems capable of powering LED-based lighting systems may be beneficial in achieving penetration into traditional white lighting markets. In fact, one draft technology roadmap for solid state white lighting indicates that power supplies and drive electronics that transform 100 Volts (AC) to 2–5 Volts (DC) with 95% efficiency should be a goal of the solid state lighting industry in order to achieve high market penetration See “Light Emitting Diodes for General Illumination II”, J. Tsao, Editor (Final Draft—Jul. 26, 2002).
Attempts have been made to design systems capable of emitting light from LEDs using an AC power source. For example, U.S. Pat. No. 5,936,599 discloses an AC powered LED array circuit for use in traffic signal displays, as well as a number of prior art circuits for similar application. In particular, the '599 patent describes a circuit having a plurality of LED pairs connected in an anti-parallel fashion to permit current flow in both halves of an AC cycle. The connection of LEDs in an anti-parallel configuration is well known. However connection of packaged LEDs in this configuration typically consumes an excessive amount of space. Moreover, the system designer may need to design complex interconnections within the luminaire to implement this design using available LED technology. A more flexible approach for designing solid state light sources for AC operation is desired.